1. Field of the Invention
The invention relates to cutting elements for rotary drill bits and particularly to cutting elements for drag-type drill bits comprising a bit body having a leading surface to which the cutting elements are fixedly mounted.
2. Description of the Prior Art
As is well known, one common form of cutting element for a rotary drag-type drill bit is a two-layer or multi-layer cutting element where a facing table of polycrystalline diamond is integrally bonded to a substrate of less hard material, such as tungsten carbide. The cutting element is usually in the form of a tablet, usually circular or part-circular. The substrate of the cutting element may be brazed to a carrier, usually also of cemented tungsten carbide, which is received in a socket in the bit body, or the substrate itself may be of sufficient axial length to be mounted directly in a socket in the bit body.
As is well known, polycrystalline diamond is formed by compressing diamond powder with a suitable binder-catalyst in a high pressure, high temperature press. In one common process for manufacturing two-layer cutting elements, diamond powder is applied to the surface of a preformed tungsten carbide substrate incorporating cobalt. The assembly is then subjected to very high temperature and pressure in a press. During this process cobalt migrates from the substrate into the diamond layer and acts as a binder-catalyst causing the diamond particles to bond to one another with diamond-to-diamond bonding, and also causing the diamond layer to bond to the substrate.
Although cobalt is commonly used as the binder-catalyst, any iron group element, such as cobalt, nickel or iron, or alloys thereof, may be employed. Polycrystalline diamond using iron group elements, or alloys thereof, as a binder-catalyst will be referred to herein as "conventional" polycrystalline diamond. Other forms of polycrystalline diamond are sometimes used as cutters in rotary drag-type drill bits, for example silicon may be used as the binder-catalyst or a conventional binder catalyst such as cobalt may be leached out of the diamond after formation. Such forms of polycrystalline diamond are not usually formed on a substrate and are generally more thermally stable than conventional polycrystalline diamond. However, problems may arise in the use of such materials as cutting elements.
When two-layer cutting elements using conventional polycrystalline diamond were first manufactured the polycrystalline diamond facing table was very thin in relation to the thickness of the substrate. More recently, however, the thickness of the diamond facing table has often been increased relative to the thickness of the substrate, particularly around the periphery of the cutting element. Such arrangements are shown, for example, in WO 97/30264. Also GB 2323110 suggests extending part of the diamond facing table through the thickness of the substrate, and up to the rear surface thereof, so that part of the diamond facing table engages the surface on which the cutting element is mounted so as to provide high modulus support (the modulus of elasticity of the diamond being greater than that of the substrate itself).
According to the present invention, the advantages provided by such arrangements are enhanced by use of cutting elements which consist entirely of conventional polycrystalline diamond material and do not incorporate an integral substrate.